Technical Abstract:
Most members of the genus Mycosphaerella grow very slowly in culture and have long latent periods in planta. Until recently, very little was known about their genetics or host-pathogen interactions. Genomes of two species have been sequenced: the septoria tritici blotch pathogen of wheat, M. graminicola; and M. fijiensis, the black Sigatoka pathogen of banana. The sequence of M. graminicola is complete; only one telomere and two internal gaps are missing. Among the 21 chromosomes of the 39.7-Mb genome, eight were smaller, had higher repetitive DNA contents, lower percent G+C and fewer genes compared to the essential chromosomes. These eight, representing 40% of the chromosomes and about 10% of the genome, could be missing in other isolates indicating an extraordinary degree of within-species genome plasticity. In contrast, the genome of M. fijiensis was 74 Mb in size but had a similar gene content. The increased size was due primarily to invasion of the genome by long-terminal repeat retrotransposons. The mitochondrial genome of M. fijiensis also was about twice the size as that of M. graminicola. Thus, extreme genome plasticity occurs both within and among species of the genus Mycosphaerella.